Since wind power energy is a power generation field having the largest occupancy rate as new renewable energy along with solar energy and is being expanded in a large capacity and offshore wind power field, a blade 3 is getting larger, such that a need for monitoring the health of the blade 3 has been increased.
FIG. 1 illustrates a general wind power generator 1. The wind power generator 1 is an apparatus that is mounted in a mountainous or at sea where wind velocity is kept to some extent to produce electricity using wind and has a rotatably controlled nacelle 4 disposed at an end of a tower 2 and a plurality of wind turbine blades 3 disposed at a rotation shaft 6 of the nacelle 4. An inside of the nacelle 4 is a generator (not illustrated) interposed in the rotation shaft 6 via a reducer (not illustrated) and is configured to allow a power generator to produce electricity while the blade 3 rotates by wind power.
In order for the wind turbine blade 3 to have high power generation capacity, the large wind turbine blade 3 is essentially required. For example, in case of the wind power generator of 750 kW, one of the lengths of the blades 3 becomes 25 m and in case of the wind power generator of 3 MW, a length of the blade 3 is suddenly increased to 45 m. As the blade 3 is large, a ratio of rigidity to high weight effectively is required. In order to effectively reduce the increase in weight and secure the rigidity ratio for coping with the increase in the size of the blade, industries have positively used a composite material.
As the composite material, glass fiber reinforced plastic (GFRP) and PVC/Balsawood are frequently used. In order to support a wind load and a self-weight, a method of reducing a weight by allowing a central portion of the blade 3 at which a shear web is located to use the GFRP having high rigidity and the remaining portion at which an aerodynamic structure to have a GFRP skin filled with the PVC or the Balsawood has been used. Further, a difference between a material thickness around a root of the blade 3 that is in particular subjected to a big load and a material thickness around a tip 5 of the blade 3 that is not subjected to a load is large.
The increase in the size of the wind turbine blade 3 essentially requires a development of a nondestructive all time defect monitoring technology. The problem of the size of the wind turbine blade 3 and the installation location of the wind power generator 1 is fundamentally impossible to perform maintenance in a laboratory in terms of disassembly. The above problem is importantly considered in that the destruction of the blade 3 due to the damage may lead to a big accident and damage other wind power generators 1 that are installed around the wind power generator 1.
The damage of the wind turbine blade 3 may occur due to several causes. The manufacturing of the wind turbine blade 3 representatively uses an infusion method of laminating a composite material, putting the laminated composite material in a mold, and permeating an adhesive resin thereinto. The damage may occur in the incomplete permeation of the adhesive resin during an infusion process and the separation of an adhesive occurring during an adhesive process of each component completed during the infusion process. Further, the damage may occur due to the external impact during a process of transporting a giant structure to an actual installation place. In addition to the defects, the damage may occur due to a peeling of the composite material according to a sudden change in a wind load during the actual driving, a crack due to a collision of an external object, and a natural disaster such as lightning, hail, typhoon, and the like.
A technology of locating acoustic emission devised to measure a tendency of damage occurrence of a pressure container, a bridge, and a concrete structure measures elastic waves propagated through a material using a plurality of sensors and searches for a damage source location using a location of a sensor and a time difference of arrival of the elastic waves. Therefore, attempts for applying a nondestructive inspection technology to the wind turbine blade 3 have been conducted recently.
FIG. 2 illustrates a method of locating a damage source using acoustic emission according to the related art. The method according to the related art roughly searches for occurrence locations of cracks by surrounding a portion to be damaged in the structure to be monitored, installing a plurality of acoustic emission (AE) sensors s1, s2, s3, and s4, detecting an acoustic emission (AE) signal, amplifying the AE signal by a signal analysis equipment C, and analyzing the AE signal.
However, the foregoing related art uses the time difference until the elastic wave reaches the AE sensors s1, s2, s3, and s4, and therefore shows satisfactory results for isotropic materials of the same material, but causes a lot of difference for a large wind turbine blade 3, and the like, due to a change in a material and a difference in a propagation speed of an elastic wave according to an elastic wave direction. That is, the related art has a problem in that it is difficult to track the damage source location in the structure formed of at least two composite materials due to greatly varying physical property values of material determining the elastic wave propagation speed.